German Patent Application No. 195 03 626 describes a conventional method where impedance values are formed from current and voltage sampling values at predetermined intervals in response to an excitation. The size of its change in time is determined from the first impedance value falling in an oscillation polygon and from the preceding impedance value, and a suspected oscillation signal is generated when the change is below a predetermined limit value. If an impedance value falling in the triggering polygon of an impedance protection is obtained in further review of the impedance values, the oscillation signal is generated if the suspected oscillation signal still exists. Oscillations with a frequency up to 1 Hz at most can be detected by this method.
German Patent No. 41 00 646 describes a method and an arrangement for protecting impedance protection devices from unwanted tripping during transient power oscillations where at least one blocking signal is generated for the impedance protection device by time derivation of a power-based electric quantity, and unwanted tripping of the impedance protection device is prevented by using an impedance locus diagram, but the resulting changes in active power and reactive power are related to the apparent power after forming the difference, and this quotient is used to form an oscillation signal.